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| United States Patent |
6,162,275
|
|
Jonsson
, et al.
|
December 19, 2000
|
Steel and a heat treated tool thereof manufactured by an integrated
powder metalurgical process and use of the steel for tools
Abstract
The invention relates to a steel having the following alloy composition in
weight-%: 1.4-1.6 (C+N), max. 0.6 Mn, max. 1.2 Si, 3.5-4.3 Cr, 1.5-3 Mo,
1.5-3 W, wherein 6<W.sub.eq <9, and W.sub.eq =% W+2.times.% Mo, 3.5-4.5 V,
max. 0.3 S, max. 0.3 Cu, max. 1 Co, a total amount of max. 1.0 of
Nb+Ta+Ti+Zr+Al, a total amount of 0.5 of other elements, including
impurities and accessory elements in normal amounts, balance iron, and
with a microstructure substantially consisting of a martensitic matrix and
in the matrix 2-15, preferably 5-10 volume-% undissolved hard products
having the particle size 0.1-3 .mu.m, said hard products being of MX-type,
where M is V and X is C and/or N, wherein 40-60% of the C and N content of
the alloy is bound to vanadium as carbides and/or as carbo-nitrides, and a
functional amount of hard products precipitated in the martensitic matrix
after solution heat treatment of the steel at a temperature between 1000
and 1225.degree. C. and tempering at least twice for at least 0.5 h at a
temperature between 190 and 580.degree. C., and the use of the steel for
tools for forming and/or cutting operations.
| Inventors:
|
Jonsson; Karin (Stocksund, SE);
Wisell; Henry (Soderfors, SE);
Westin; Leif (Soderfors, SE)
|
| Assignee:
|
Erasteel Kloster Aktiebolag (SE)
|
| Appl. No.:
|
331117 |
| Filed:
|
June 17, 1999 |
| PCT Filed:
|
February 25, 1998
|
| PCT NO:
|
PCT/SE98/00334
|
| 371 Date:
|
June 17, 1999
|
| 102(e) Date:
|
June 17, 1999
|
| PCT PUB.NO.:
|
WO98/40180 |
| PCT PUB. Date:
|
September 17, 1998 |
Foreign Application Priority Data
| Current U.S. Class: |
75/238; 75/236; 75/244; 75/246; 419/11; 419/29; 419/49 |
| Intern'l Class: |
C22C 033/02 |
| Field of Search: |
75/236,244,238,246
419/11,49,29
|
References Cited
U.S. Patent Documents
| 3809541 | May., 1974 | Steven.
| |
| 5435827 | Jul., 1995 | Wisell.
| |
| 5522914 | Jun., 1996 | Stasko et al.
| |
| 5525140 | Jun., 1996 | Wisell.
| |
| 5578773 | Nov., 1996 | Wisell.
| |
| Foreign Patent Documents |
| 88/07093 | Oct., 1988 | WO | .
|
| 93/02819 A1 | Feb., 1993 | WO | .
|
| 93/02818 | Feb., 1993 | WO | .
|
Primary Examiner: Mai; Ngoclan
Attorney, Agent or Firm: Kilpatrick Stockton LLP
Claims
What is claimed is:
1. A powder-metallurgically manufactured steel alloy for tools for forming
and/or cutting operations, the alloy comprising in weight-%:
1.4-1.6 (C+N);
max. 0.6 Mn;
max. 1.2 Si;
3. 5-4.3 Cr;
1.5-3 Mo;
1.5-3 W, wherein 6<W.sub.eq <9, and W.sub.eq =% W+2.times.% Mo;
3.5-4.5 V;
max. 0.3 S;
max. 0.3 Cu;
max. 1 Co; and
a total amount of max. 1.0 of Nb+Ta+Ti+Zr+Al, balance essentially only
iron, impurities and accessory elements in normal amounts.
2. The steel alloy of claim 1, comprising at least 1.44 and at most 1.56
(C+N).
3. The steel alloy of claim 1 wherein 40-60% of C and N exist in
undissolved hard products of MX-type, which means primary carbides or
carbo-nitrides, where M is V and X is C and/or N.
4. The steel alloy of claim 1, comprising max. 0.03 S.
5. The steel alloy of claim 1, comprising 0.1-0.3 S.
6. The steel alloy of claim 1, comprising 3.8-4.2 Cr.
7. The steel alloy of claim 1, wherein 6.5.ltoreq.W.sub.eq .ltoreq.8.5.
8. The steel alloy of claim 1, comprising 3.8-4.2 V.
9. A tool made of the steel alloy having a composition of claim 1, the tool
material having a micro-structure substantially consisting of a
martensitic matrix and in the matrix 2-15 volume-% of undissolved hard
products having the particle size 0.1 -3 .mu.m, said hard products being
of MX-type, where M is V and X is C and/or N, wherein 40-60% of the C and
N content of the alloy is bound to vanadium as carbides and/or as
carbo-nitrides, and a functional amount of hard products precipitated in
the martensitic matrix after solution heat treatment of the steel at a
temperature between 1000 and 1225.degree. C. and tempering at least twice
for at least 0.5 h at a temperature between 190 and 580.degree. C.
10. The tool according to claim 9, wherein the martensitic matrix contains
a functional amount of hard products of M.sub.2 X-type, where M is metals
belonging to the group consisting of Cr, Mo, W, V, and Fe, and X is C and
N, said hard products having a size smaller than 100 nm, obtainable by
tempering the steel at a temperature between 520 and 570.degree. C.
11. The tool according to claim 9, wherein the tool material contains a
functional amount of hard products of M.sub.3 X-type, where M
substantially is Fe and Cr, and X is C and/or N, obtainable by tempering
the steel at a temperature between 190 and 250.degree. C. after solution
heat treatment at a temperature between 1000 and 1100.degree. C.
12. The tool according to claim 9, wherein tool material has a hardness of
at least 62 HRC and a bending strength of at least 5.5 kN/mm.sup.2 after
hardening from a temperature between 1100 and 1200.degree. C. and
tempering at a temperature between 520 and 570.degree. C.
13. An integrated process for the manufacturing of a steel and a tool
thereof, comprising the following steps:
preparing a steel melt having the alloy composition of claim 1;
forming droplets of the melt, and cooling the droplets to form a powder of
said steel alloy, in which existing hard products of type MX, where M
substantially is V, and X is C and/or N, consist of particles, in which at
least 90% of the total amount of said hard products has a particle size
between 0.1 and 3 .mu.m;
densifying the powder to form a body with complete density through a
densification process which comprises hot isostatic compaction;
hot working the body through forging and/or rolling;
soft annealing the forged and/or hot roll product;
making a tool with desired shape of the soft annealed product; and
hardening the tool through solution heat treatment (austenitisation) at a
temperature between 1000 and 1225.degree. C., intensified cooling to below
500.degree. C. and continued cooling to below 50.degree. C., and tempering
at a temperature between 190 and 580.degree. C., such that the tool
material will obtain a micro-structure substantially consisting of a
martensitic matrix and in the matrix 2-15 volume-% of undissolved hard
products having the particle size 0.1-3 .mu.m, said hard products being of
MX-type, where M is V and X is C and/or N, wherein 40-60% of the C and N
content of the alloy is bound to vanadium as carbides and/or as
carbo-nitrides, and a functional amount of hard products precipitated in
the martensitic matrix after said solution heat treatment cooling and
tempering of the steel.
14. The steel alloy of claim 7, wherein 7.ltoreq.W.sub.eq .ltoreq.8.
15. The tool of claim 9, in which the martensitic matrix of the tool
material contains 5-10 volume-% of undissolved hard products having the
particle size 0.1-3 .mu.m, said hard products being of MX-type, where M is
V, and X is C and/or N.
Description
TECHNICAL FIELD
The invention relates to a powder-metallurgically manufactured steel for
tools, particularly for so called cold work tools, for forming and/or
cutting operations. The invention also relates to the tool that is made of
the steel and which has attained specific, desired features through a heat
treatment which has been adapted the alloy composition and to the
powder-metallurgical manufacturing technique. The invention also relates
to the integrated process for the manufacturing of the steel, the tool,
and the heat treatment of the tool, wherein the expression "integrated"
shall mean that the powder-metallurgical manufacturing technique as well
as the heat treatment of the tool contribute to the achievement of the
desired combination of features of the finished tool.
BACKGROUND OF THE INVENTION
Steels of the type indicated in the preamble usually are referred to as
cold work steels. Dies for cold extrusion of metals; deep drawing and
powder pressing counter dies; knives and other tools for shearing and
cutting, etc., are typical applications of cold work steels. A
powder-metallurgically manufactured high speed steel having the
composition 1.28 C, about 0.3 Si, about 0.5 Mn, 4.2 Cr, 5.0 Mo, 6.4 W, 3.1
V, balance Fe and impurities, is a well known steel for this type of
applications. A drawback of this steel is that it does not have a
toughness that satisfies highest demands. Another powder-metallurgically
manufactured steel known in the art has the composition 1.5C, 1.0 Si, 0.4
Mn, 8.0 Cr, 1.5 Mo, 4.0 V, balance Fe and impurities. This steel also
after tempering has a comparatively high content of rest austenite, which
is attributed to the high chromium content, which reduces the hardness.
Therefore it is a long felt demand of a material which combines the best
features of the said steels. More particularly, this can be expressed such
that there is a demand of a steel which affords optimal features as far as
toughness, wear resistance and hardness for the intended field of use are
concerned at the same time as the total content of alloy elements, and
particularly the most exclusive alloy elements, are kept at a
comparatively low level in order to make the material favourable also from
a cost point of view.
BRIEF DISCLOSURE OF THE INVENTION
It is the purpose of the invention to satisfy the above mentioned demands.
This can be achieved therein that the invention is characterized by what
is stated in the appending claims. Without binding the invention to any
specific theory, the importance of the various alloy elements and the
various structure constituents for the achievement of the desired
combination of features shall be explained more in detail. As far as
percentages are concerned, alloy contents are always measured in weigh-%
and structure constituents in volume-% if not anything else is stated.
Carbon and nitrogen
Carbon and nitrogen shall exist in an amount of at least 1.4% and not more
than 1.6%, preferably at least 1.44% and not more than 1.56%; typically
1.5%. Normally, the nitrogen content amounts to not more than 0.1%, but
the powder-metallurgical manufacturing technique makes it possible to
dissolve as much as about 1% nitrogen, if the carbon content is so low
that the total amount of carbon and nitrogen is 1.4-1.6%. A variant of the
steel therefore is characterized in that the steel contains a high content
of nitrogen, max. 1.0%, e.g. 0.3-1.0% N, which can be achieved through
solid phase nitriding of produced powder, wherein the nitrogen can replace
carbon in those hard components which shall exist in the steel in the
final tool. Thus 40-60% of the carbon 20 and the nitrogen shall be
included in undissolved hard components of MX-type, i.e. primary carbides
or carbo-nitrides, where M is substantially vanadium and X is carbon
and/or nitrogen, while the rest essentially is dissolved in the matrix or
is present as precipitated hard components. Lower contents than 1.4%
carbon +nitrogen do not afford sufficient hardness and wear resistance,
while higher contents than 1.6% can cause embrittlement problems.
Manganese
Manganese is present in amounts which are normal for these types of steel,
i.e. from at least 0.1% up to not more than about 0.6%. The typical
manganese content is about 0.3%.
Silicon
Silicon is present in an amount of at least 0.1% and can exist in amounts
up to about 1% or not more than 1.2% in a silicon alloyed variant, but
normally the steel does not contain more than 0.6% silicon or typically
about 0.5% silicon.
Sulphur
Sulphur normally is not present more than as an impurity in the steel, i.e.
in an amount not more than 0.03%. In order to improve the cutability of
the steel, however, up to 0.3 sulphur can be added in a sulphur alloyed
variant. In this case, the steel contains 0.1-0.3% sulphur.
Chromium
Chromium shall be present in an amount of at least 3.5% in order to afford
a sufficient hardness to the steel. The content of chromium, however, must
not exceed 4.3%. If the chromium content is higher, there is a risk,
especially at comparatively low solution temperatures, that existing
chromium carbides in the steel will not be dissolved. The chromium
carbides which are concerned in this connection are of M.sub.7 C.sub.3 -
and M.sub.23 C.sub.6 -type, which are not desired. Moreover, the
precipitation of M.sub.2 C-carbides or corresponding in the martensite
which is formed at the cooling from the tempering temperature, which
precipitation is desired according to the invention, will be detrimentally
influenced by the chromium content when rest austenite is transformed to
martensite. At higher chromium contents there is a risk that the rest
austenite content will be higher than what is desirable. Not only would
this rest austenite have an impact upon the precipitation of M.sub.2
C-carbides or corresponding but it would also per se be undesired, because
it would reduce the hardness which can cause plastic deformation e.g.
deformation of sharp corners or edges on the tool when the tool is used.
Molybdenum and tungsten
Each of molybdenum and tungsten shall exist in the steel in an amount of at
least 1.5% but not more than 3%. Preferably each of the said elements
shall exist in an amount of 1.8-2.8%, suitably 2.1-2.7%, typically 2.5%.
However, W.sub.eq =% W+2.times.% Mo shall be at least 6 and not more than
9, preferably at least 6.5 and not more than 8.5, suitably at least 7 and
not more than 8, typically 7.5. The lowest content of W.sub.eq is required
in order to obtain a desired precipitation of M.sub.2 C-carbides or
corresponding (nitrides, carbo-nitrides) in connection with the high
temperature tempering which shall be described in the following, while the
maximal content is chosen in order to avoid the formation of primary
M.sub.6 C-carbides, i.e. W, Mo-carbides which are not desirable according
to the invention. By maximizing the total content of molybdenum and
tungsten in this way, the content of M.sub.6 C-carbides and corresponding
can be maximized to 2%, preferably max. 1%. As a matter of fact, any
detectable M.sub.6 C-carbides or corresponding are normally not present in
the steel of the invention.
Vanadium
Vanadium shall exist in an amount of at least 3.5% in order that the steel
shall get a desired wear resistance through a high content of MC-carbides
or corresponding carbo-nitrides. The maximum content may amount to 4.5%.
The toughness will be too low if the vanadium content is higher.
Other carbide and nitride formers
The steel of the invention does not contain any intentionally added carbide
or nitride formers besides the mentioned carbide and nitride formers and
iron. The total amount of niobium, tantalum, titanium, zirconium, and
aluminium, and possible further strong carbide and/or nitride formers
amounts to totally max. 1.0%.
Cobalt
The cobalt is an element which generally increases the steel's hardness. It
is not intentionally added to the steel of the invention but can exist as
a component in used raw materials and this particularly may be the case
when the steel is manufactured in plants having a main production of high
speed steels, and can be tolerated in amounts up to max. 1%.
Other elements
The steel of the invention should not contain any further, intentionally
added alloy elements. Copper may exist in an amount up to max. 0.3%, tin
in an amount up to max. 0.1%, lead up to 0.005%. The total content of
these and other elements in the steel, except iron, may amount to max.
0.5%.
Manufacturing and treatment of the steel and its microstructure
A melt having the alloy composition of the invention is prepared. A stream
of molten metal is disintegrated to very small droplets by means of an
inert gas which can be argon or nitrogen. Nitrogen is particularly used if
the steel shall be intentionally alloyed with nitrogen. The droplets are
cooled as they fall though the inert gas and solidify to a fine powder.
The composition in each individual powder grain will be very homogenous,
because segregation do not have time to establish during the course of
solidification. In the powder grains, however, there exist precipitated
primary MC-carbides, or carbo-nitrides when the powder grains contain a
high content of nitrogen. About half the amount or 40-60% of the total
content of carbon and nitrogen is collected in the MC-carbides, or
corresponding carbo-nitrides, where M is vanadium. These carbides or
carbo-nitrides have a particle size which does not exceed 3 .mu.m, and at
least 90% of the total amount of these hard products have sizes in the
size range 0.1-3 .mu.m.
The powder is sieved and charged in metal sheet capsules which are gas
evacuated and then sealed, whereupon the capsules with their content first
is cold compacted and then subjected to hot isostatic pressing, so called
HIP-ing, at a temperature above 900.degree. C., normally in the range
900-1200.degree. C., and at a pressure over 90 MPa, normally in the range
90-150 MPa. The material then is forged and rolled to desired shape and
dimension in a conventional way. After finished hot working, the material
is soft annealed at a temperature of about 900.degree. C. and is then
slowly cooled.
The material is delivered in the soft annealed condition to tool makers of
different direction. Tool makers namely is a heterogeneous group of
manufacturers. It is in the first place the facilities for the heat
treatment of the finished tools that differ very much, which has to do
with such factors as the degree of specialisation of the tool makers, the
age of the plant, etc.
Basically, there are two main types of plants, namely on one hand plants on
which it is possible and conventional to harden the steel from high
solution heat treatment temperatures, which means temperatures in the
range 1100-1225.degree. C., and on the other hand plants in which the
furnaces do not allow higher temperatures than 1000-1100.degree. C. for
the solution heat treatment. In the first place high speed steel tool
makers belong to the first group, while manufacturers of conventional cold
work steel tools belong the latter group. It is a purpose of the invention
to satisfy both these categories. According to the broadest aspect of the
invention, the manufactured tools are hardened through solution heat
treatment at a temperature between 1000 and 1225.degree. C. followed by
rapid cooling to below 500.degree. C. in order to prevent formation of
pearlite and/or bainite, whereafter the cooling can proceed at a slower
rate by cooling in air to room temperature or at least to below 50.degree.
C. The material then is tempered at a temperature between 190 and
580.degree. C. at least twice, each time for at least half an hour but
normally not for a longer period of time than 4 h in connection with each
tempering operation.
The result in terms of the micro-structure of the material and hence also
in terms of the mechanical characteristics of the material depends on
within which part of the said temperature ranges for the solution heat
treatment, and for the tempering, that the tool maker operates. In the
first case--the high temperature alternative--it is possible to choose a
hardening temperature (solution heat treatment temperature) within a
comparatively broad temperature range, usually within the range
1050-1250.degree. C. depending on which hardness of the end product that
is desired after tempering. For the tempering operation, however, a more
narrow temperature range is applied in order that an aimed secondary
hardening effect shall be achieved, namely a temperature between 520 and
580.degree. C. The MC-carbides and/or corresponding carbo-nitrides are
only partially but essentially all other carbides and nitrides are
completely dissolved during the solution heat treatment. The degree of
dissolution of the MC-carbides depends on the solution heat treatment
temperature. At the intensified cooling there is formed martensite, which
is the dominating constituent of the matrix. In the latter there is 2-15,
preferably 5-10 vol-% undissolved MC-carbides or corresponding
carbo-nitrides. However, also after the cooling operation there remains a
certain amount of rest austenite. The tempering at 520-580.degree. C.,
normally at 550-560.degree. C. aims at transforming the rest austenite to
martensite and to provide precipitations of M.sub.2 C-carbides and/or
corresponding carbo-nitrides in the martensite. In order to secure that
essentially all rest austenite is transformed to martensite, the tempering
is carried out twice or more times. The precipitated M.sub.2 C-carbides or
corresponding have a size smaller than 100 nm. The typical size lies,
according to previously made and published studies, in the size range 5-10
nm. They are in other words sub-microscopic and can therefor not be
observed by means of conventional microscopes. They are, however,
recognised through the secondary hardening that is achieved by the
tempering operation, which secondary hardening is something that is
characteristic for this type of precipitation. Therefor it can implicitly
be established that M.sub.2 C-carbides do exist in large amounts in the
martensitic matrix of the material of the invention. It is, however, not
within the frame of the development work of the invention to quantify the
amount of precipitated M.sub.2 C-carbides, where M can represent any
carbide forming metal in the alloy, such as tungsten, molybdenum,
chromium, iron and vanadium, but generally speaking can be stated that the
number of small M.sub.2 C-carbides widely exceeds e.g. 1000
carbides/.mu.m.sup.2. Even if other metals than tungsten and molybdenum
are parts of the M.sub.2 C-carbides, the said elements are essential
ingredients. That is one of the reasons why W.sub.eq shall be at least 6,
preferably at least 6.5 and suitably at least 7% in the steel. Besides
undissolved MC-carbides and/or corresponding carbo-nitrides and the
secondary precipitated M.sub.2 C-caribdes and/or carbo-nitrides, the
tempered material does not contain any other carbides to any substantial
degree. Thus, the material is void of chromium carbides, and M.sub.6
C-carbides do not either exist in any noticeable degree.
As far as the low temperature alternative is concerned, the solution heat
treatment is performed at a temperature between 1000 and 1100.degree. C.,
while the tempering typically is performed at a temperature between 190
and 250.degree. C., more particularly between 190 and 220.degree. C. The
solution heat treatment corresponds to the solution heat treatment at the
high temperature alternative, within the lower part of the wider range as
mentioned above, which implies that a minor dissolution of the MC-carbides
and a substantially total dissolution of all other carbides are achieved.
The cooling is carried out in the same mode as according to the foregoing
alternative. The tempering is carried out twice or more times for at least
half an hour each time. M.sub.2 C-carbides are not precipitated and nor is
there achieved the same pronounced secondary hardening effect at this low
temperature tempering. Instead M.sub.3 C-carbides are precipitated, which
substantially consist of cementite. A certain amount of rest austenite,
max. 20%, preferably max. 15%, is not transformed to martensite but exists
as part of the matrix in the finished tool according to this alternative.
This to some degree reduces the hardness of the material, but on the other
hand, the amount of remaining, undissolved MC-carbides is greater than
after the high temperature tempering, which improves the wear resistance.
The alternative which includes the lower solution heat treatment
temperature and the lower tempering temperature therefor may be a more
advantageous heat treatment for certain types of tools, depending on their
field of use, or desirable depending on limited access to furnaces with
about 100.degree. C. as highest possible temperature.
BRIEF DESCRIPTION OF DRAWINGS
The invention shall be explained more in detail with reference to performed
experiments and achieved results. Herein reference will be made to the
accompanying drawings, in which
FIG. 1 shows the hardness versus the hardening temperature after high
temperature tempering of a steel according to the invention and of a
reference material;
FIG. 2 shows the bending strength--tensile strength--versus the hardening
temperature of the steel of the invention for two alternative tempering
temperatures and also for a reference material;
FIG. 3 shows the bending strength--deflection--versus the hardening
temperature for the same materials and during the same conditions as for
FIG. 2;
FIG. 4 shows the wear resistance of a number of examined steels;
FIG. 5 shows the toughness in terms of impact strength for a number of
tested steels;
FIG. 6 illustrates the content of MC-carbides in a steel of the invention
and the content of MC-carbides and M.sub.6 C-carbides in an other material
after tempering at different solution heat treatment temperatures;
FIG. 7 shows the micro-structure of a steel of the invention after heat
treatment; and
FIG. 8 shows a typical tool for which the steel of the invention can be
used.
DESCRIPTION OF CARRIED OUT EXPERIMENTS
In a first series of experiments seven alloy variants were made, steels No.
1-7 in Table 1. Powders were made of the molten alloys according to the
technique which has been described in the foregoing brief disclosure of
the invention. The powder was filled in small metal sheet capsules, .O
slashed. 46 mm, length about 0.5 m. The capsules were closed and gas
evacuated, whereafter the capsules with their content were compacted to
full density, comprising hot isostatic pressing at a temperature of
1150.degree. C. and a pressure of 100 MPa.
TABLE 1
__________________________________________________________________________
Composition, weight-%, balance Fe and unavoidable impurities
__________________________________________________________________________
C Si Mn Cr
Steel No.
Typical
Analysed
Typical
Analysed
Analysed
Typical
Analysed
__________________________________________________________________________
1 1.1 1.09 0.5 0.49 n.a. 4.0 3.97
2 1.2 1.19 0.5 0.50 " 4.0 3.87
3 1.3 1.30 0.5 0.50 " 4.0 3.99
4 1.4 1.42 0.5 0.53 " 4.0 3.97
5 1.5 1.50 0.5 0.52 " 4.0 3.96
6 1.6 1.59 0.5 0.55 " 4.0 3.99
7 1.3 1.29 1.0 1.02 " 4.0 4.05
8 1.5 1.48 0.5 0.55 " 4.0 4.00
9 1.28
n.a. .about.0.3
n.a. .about.0.5
4.2 n.a.
10 0.8 " .about.0.4*
" .about.0.4*
4.0 "
11 1.5 " 1.0 " 0.4 8.0 "
12 0.9 " 0.3 " 0.3 4.2 "
13 1.55
" 0.4 " 0.4 12.0
"
__________________________________________________________________________
Mo W V N
Steel No.
Typical
Analysed
Typical
Analysed
Typical
Analysed
Analysed
__________________________________________________________________________
1 2.0 2.00 2.0 2.04 4.0 4.02 n.a.
2 2.0 1.99 2.0 2.06 4.0 4.00 "
3 2.0 1.99 2.0 2.07 4.0 3.99 "
4 2.0 1.96 2.0 2.10 4.0 4.03 "
5 2.0 1.95 2.0 2.08 4.0 3.91 "
6 2.0 1.97 2.0 2.13 4.0 4.04 "
7 2.0 2.02 2.0 2.08 4.0 4.02 "
8 2.5 2.50 2.5 2.50 4.0 3.98 "
9 5.0 n.a. 6.4 n.a. 3.1 n.a. "
10 3.0 " 3.0 " 3.0 " "
11 1.5 " -- " 4.0 " "
12 5.0 " 6.4 " 1.8 " "
13 0.8 " -- " 0.8 " "
__________________________________________________________________________
*Estimated values
n.a. = not analysed
After the hot isostatic pressing the samples were not subjected to any heat
treatment as distinguished from what is normal for full scale production.
Instead, each HIP-ed capsule was cut to pieces for heat treatment
according to Table 2.
TABLE 2
______________________________________
Heat treatment schedule
Solution heat treatment temperature, .degree. C., at the hardening
Tempering 1000 1050 1100 1150 1180 1200 1220
______________________________________
200.degree. C., 2 .times. 2 h
500.degree. C., 3 .times. 1 h
520.degree. C., 3 .times. 1 h
540.degree. C., 3 .times. 1 h
550.degree. C., 3 .times. 1 h
560.degree. C., 3 .times. 1 h
580.degree. C., 3 .times. 1 h
600.degree. C., 3 .times. 1 h
______________________________________
Hardness and grain sizes of the hardened and tempered samples were
measured. The grain size varied between 7 and 10 .mu.m for those samples
which had been hardened from at the lowest 1150.degree. C. The hardnesses
varied depending on the carbon content. By choosing the carbon content
1.5% C there was achieved a maximal hardness of about 64 HRC after
tempering. It was, however, estimated that the total amount of molybdenum
and tungsten was a little too low in order that secondary hardening should
be achieved to a desirable degree through precipitation of M.sub.2
C-carbides after high temperature treatments at a tempering temperature of
about 560.degree. C. which is optimal for such precipitation hardening.
Therefor there was produced, for further studies, a heat with the aimed
analysis (typical composition) 1.50C, 4.2 Cr, 2.5 Mo, 2.5 W, 4.0 V, normal
amounts of Mn and Si, balance Fe and unavoidable impurities. The analysed
composition is given in Table 1, steel No. 8. Also the typical
compositions of a number of reference materials, steels Nos. 9-13, have
been included in Table 1.
About 6 tons of powder were made of steel No. 8. The powder was filled in
capsules, each containing about 1500 kg powder. The capsules were closed,
gas evacuated, cold and hot isostatic compacted at a temperature of
1150.degree. C. and a pressure of 100 MPa, forged, and rolled to the shape
of rods, some of them all the way down to the dimension .O slashed. about
6.2 mm. Test specimens were machined to the size .O slashed. 6 mm. Equal
test specimens also were made of steel No. 9.
The test specimens were hardened from different solution heat treatment
temperatures, varying between 1000 and 1200.degree. C., and tempered
3.times.1 h at 560.degree. C. The results are given in FIG. 1, which shows
that the substantially higher alloyed reference material No. 9 had the
highest hardness but also that steel No. 8 of the invention achieved a
hardness which is sufficient for the intended applications.
Thereafter the toughness was examined after different solution heat
treatment temperatures for steel No. 8 of the invention after tempering on
one hand at 560.degree. C., 3.times.1 h, and on the other hand after
tempering at 200.degree. C., 2.times.2 h and for the reference material,
steel No. 9, after the same tempering treatment as at the hardness test,
i.e. at 560.degree. C., 3.times.1 h. The toughness was measured in terms
of bending strength/tensile strength and in terms of bending
strength/deflection. The results are illustrated in FIG. 2 and FIG. 3. The
bending strength tests show that the steel of the invention had the
highest toughness regardless of solution heat treatment temperature.
Further FIG. 2 shows that best toughness after solution heat treatment at
temperatures between 1050 and 1200.degree. C. and higher was achieved
after high temperature tempering treatment, i.e. according to the example
at 560.degree. C., but that after solution at lower temperatures,
1000-1050.degree. C., best toughness was achieved after tempering
treatment within the lower temperature range, according to the example at
200.degree. C.
The same tendency is illustrated also in FIG. 3, but it is here much more
evident that by far the best toughness is achieved with the steel of the
invention after the high temperature annealing treatment.
For wear resistance tests, test specimens were used, size .O slashed. 15
mm. The tests were carried out according to the method which is known in
the art as the "Pin on disc, dry SiO.sub.2 flint paper"--test, grain size
150 mesh, load 20 N, 2 min. Also the steels which in Table 1 are
denominated steel Nos. 11, 12, and 13 were tested besides steel No. 8 of
the invention and the reference steel No. 9. Steel No. 11 was a
powder-metallurgically manufactured cold work steel; steel No. 12 was a
conventionally manufactured high speed steel, type M2; and steel No. 13
was a conventional cold work steel, type D2. The hardnesses are given in
FIG. 4. Steel No. 8 of the invention was tested on one hand after high
temperature tempering at 560.degree. C. and on the other hand after low
temperature tempering at 200.degree. C.
As far as the interpretation of the bar chart in FIG. 4 is concerned, the
wear resistance is proportional to the height of the bar. Best result was
achieved for steel No. 8 after hardening from 1060.degree. C. and
tempering 2.times.2 h at 200.degree. C., and next best was steel No. 8 of
the invention when hardened from 1150.degree. C. and tempered 3.times.1 h
at 560.degree. C. Equal wear resistance had the cold work steel No. 13,
which is a conventionally manufactured high chromium steel with a high
amount of large chromium carbides which promote the wear resistance but
which on the other hand impair other important features, particularly the
toughness.
Then the impact strength according to the VW-method (Volkswagen), specimen
size 7.times.10.times.55 mm, was investigated for steel Nos. 8-13. The
applied heat treatments and achieved results are given in Table 3. The
results are also illustrated in FIG. 5, which shows that steel No. 8 of
the invention had the by far best toughness results in terms of impact
strength among the tested steels.
TABLE 3
______________________________________
Impact strength tests, VW-method
Austenitising
Tempering temperature, Impact
Steel
temperature,
.degree. C., number of tempering
Hardness,
Strength,
No. .degree. C.
operations, and duration
HRC Joule
______________________________________
8 1020 200, 2 .times. 2 h
61,8 111
8 1020 525, 2 .times. 2 h
60,5 100
8 1020 560, 2 .times. 1 h
58,7 120
8 1100 560, 3 .times. 1 h
61,3 90
9 1075 560, 3 .times. 1 h
61,1 61
11 1020 200, 2 .times. 2 h
59,7 88
11 1020 525, 2 .times. 2 h
58,2 74
12 1050 560, 2 .times. 1 h
57,7 36
13 1020 200, 2 .times. 2 h
59,4 34
______________________________________
Finally also the carbide content in the steel of the invention was examined
after cooling from different solution heat treatment temperatures. As a
reference also the carbide content in a known valve steel--steel No. 10 in
Table 1--was determined, the said steel having a lower carbon content and
somewhat lower vanadium content than the steel of the invention. The total
amount of molybdenum and tungsten, expressed as W.sub.eq, corresponded
with what can be tolerated at a maximum according to the broadest W.sub.eq
range according to the invention. The study showed, FIG. 6, that only
MC-carbides could be detected in the steel of the invention, more
particularly between 5 and 10% within the entire tested temperature range.
Steel No. 10 contained less than 5% MC-carbides but also M.sub.6
C-carbides after hardening from temperatures up to at least about
1150.degree. C.
FIG. 7 shows the micro-structure of steel No. 8 of the invention after
hardening from 1100.degree. C., tempering 3.times.1 h, 560.degree. C. The
bright, round or more or less oval particles consist of undissolved
MC-carbides. The matrix consists of tempered martensite. Secondarily
precipitated M.sub.2 C-carbides, which exist in a large amount in the
martensitic matrix are not visible at the actual magnification because of
their smallness; sizes in the order 5 a 10 nm.
In FIG. 8 there is shown a tool, an upper-die a, intended to form part of a
punching tool for which the steel of the invention advantageously can be
used.
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